I recently purchased Dorico 4 Pro as a crossgrade from Sibelius Ultimate which I was running on a monthly subscription.
Does my purchase of a crossgrade disqualify me from creating a new monthly subscription to Sibelius as it appears my old Sibelius subscription has expired or been cancelled
Avid is not going to stop you using Sibelius because you’ve bought a competing product!!
A cross-grade is just an enticement to encourage users of other products to try Dorico. You can continue to use Sibelius, and/or Finale after purchase.
I seem to recall a time when I had to mail in my Encore floppies to get a discount for Finale.
I imagine that was before emailing a screenshot was a thing!